Communications cables typically comprise a signal-conducting core surrounded by a protective sheath. The core can, for example, conduct light signals or electrical signals. In many cables the space between the conductor and sheath contains a filler, the role of which is to protect and cushion the core from external forces that might be produced by, for example, bending or coiling, particularly in the case of fibre-optic cables. A further role of the filler is the prevention of water ingress which is particularly pertinent should the core comprise a metal such as copper. In order to fulfil these requirements, the filler must display a number of characteristics. The filler must be of sufficient viscosity in order to allow lateral movement of the core which occurs during, for example, bending, coiling or laying. The viscosity must however not be so low as to allow a drip wise loss of filler during vertical laying of cables. Moreover, this balance of properties must be maintained over a temperature range of −40 to +80° C. The filler must be formulated to be chemically compatible with cable grade polymers, which includes not only the cable sheath but also coatings typically found on optical fibres. The filler should also show a high degree of elasticity in order to absorb the force of impacts that the cable sheath may undergo during its operating lifetime. Relatively high ambient temperatures can be reached through fabrication of such cables resulting in thermal expansion of the filler which then leads to the formation of holes and cavities on cooling. Such holes and cavities can potentially become a water path which in fibre optic cables can lead to attenuation of the light wave guide. Thus cable fillers should ideally show low thermal conductivity. For electrical applications or cores transmitting electrical signals, it is advantageous if the filler has a low permitivity thus insulating the conducting core. This has the additional benefit of rendering the filler hydrophobic thereby protecting the core from water ingress. The anti-drip resistance of fillers can be improved by reducing their specific weight. Finally, for easy handling, it is preferred if the filler is semi-dry to the touch, rather than sticky.
Existing fillers used in telecommunication cables include oil gels which are primarily blends of oils and gelling agents. In application, they penetrate between bundles of for example, densely packed insulated copper conductors and in so doing insulate them from moisture. The oil, which comprises a major part of the blend, can be a naphthenic or paraffinic processing oil, a mineral oil, a synthetic product such as a polybutane or a silicone oil. Gelling agents include waxes, silicic (silica gels) acids, fumed silica, fatty acid soaps and thermoplastic elastomers. Typically the gelling agent comprises less than 10% of the whole.
One particular family of thermoplastic elastomers marketed under the trade mark ‘Kraton’ (Shell Chemical Company), comprises di-block, tri-block or multi-atm molecular configurations of rubber and polystyrene segments.
U.S. Pat. No. 5,657,410 describes an optical transmission element which includes a filler comprising between 80% and 95% by weight of a monomeric plasticizer having a molecular weight in the range 200-2000 grams per mole. Such monomeric plasticizers include esters of phthalates, trimellitates, phosphates and fatty esters. Additional substances may also be added such as thickeners. The thickener can take the form of small spheres. Hollow spheres are preferred due to their great compressibility and easy processibility. Thixotropic agents may also advantageously be added. They include finely divided or fumed silica, alumina, and bentonites as well as mixtures of these substances.
The use of micro spheres, e.g. hollow micro spheres, in cable filling compounds is also described in U.S. Pat. No. 5,698,615. In this disclosure the cable filler comprises a substantially dry hydrophilic composition containing inter alia, in addition to the micro spheres, water absorbent swellable powder particles, preferably of particle size range 1-30 μm and a “parting powder” having particles of preferably 1/100th the size of the swellable powder particles. Due to the hydrophilic nature of the swellable powder particles, the composition always retains some water. The parting powder particles are disposed between the swellable powder particles to prevent agglomeration as the swellable polymer particles absorb water. Suitable swellable powder particles are those based on the polyacrylic acid sodium salt. The parting powder particles are typically inorganic powders such as talcum, mica, graphite and silicates. The absorption of water by the swellable powder particles transforms the dry composition into a gel which seals the core from further water ingress. The compositions can also contain a small amount of an oil or an adhesive to reduce any potential dust hazard.
WO 99/15582 discloses a composition which includes expandable hollow micro spheres for use in encapsulation of for example semi-conductor chips. Such hollow micro spheres, similar in morphology to the micro spheres disclosed in U.S. Pat. No. 5,657,410 and U.S. Pat. No. 5,698,615, comprise a polymeric shell encapsulating a blowing agent. When heated, the polymeric shell of the expandable micro spheres gradually softens and the liquid blowing agent, typically isobutane, starts to evaporate thus expanding the microsphere.
A light wave guide lead is disclosed in U.S. Pat. No. 5,335,302 which comprises at least one light wave guide accommodated in a protective sheath and embedded in a pasty filling material containing small micro spheres. The small micro spheres, which can be solid and rigid, solid and elastic or hollow and elastic, are included as fillers to reduce the cost, and to provide improved rheological and cushioning properties. A preferred filler comprises an oil, a thixotropic agent (for example fumed silica) and an unspecified organic thickener.